Hunting for planets around nearby stars

The UK ATC was invited to participate in the Royal
Society's prestigious Summer Exhibition 2004.
An introduction to the science behind the exhibit
is given below. The posters that supported
the exhibit are also available through the menu on the left.

Other solar systems hidden in Stardust

Artistâs impression of the Vega planetary system. The Neptune-like planet can be seen in the foreground with the Jupiter-like planet orbiting nearer the star.
The two clumps of dust are visible at 90Âº to the position of the Neptune-like planet.
Credit: David A Hardy/UKATC

Astronomers at the UK Astronomy Technology Centre at the Royal Observatory,
Edinburgh have produced compelling evidence of a planetary system which is
more like our own Solar System than any other so far discovered. It is around
Vega, one of the brightest stars in the sky.

'Neptune' is the key

The SCUBA image: This is a false colour image of the heat emitted from the dust disk around Vega.
The image shows the disk seen face-on. The disk structure includes two bright clumps, represented
by the yellow and red colours. The star is barely noticeable and is located at the centre of the
image, mid-way between the two clumps. The position of the star (*) and the predicted position and
direction of the planetâs obit (+) are marked.

New computer modelling techniques have shown that observations of the structure
of a faint dust disk around Vega can be best explained by a Neptune-like
planet orbiting at a similar distance to Neptune in our own solar system.
The wide orbit of the Neptune-like planet means that there is plenty of
room inside it for small rocky planets similar to the Earth – the
Holy Grail for astronomers wanting to know whether we are alone in the
Universe.

The modelling is based on observations taken with
the world's most sensitive submillimetre camera, SCUBA. The camera, built
in Edinburgh, is operated
on the James Clerk Maxwell Telescope in Hawaii. The SCUBA image shows a
disk of very cold dust (-180ºC) in orbit around the star.

Clumps in the disk

Prediction: The modelling is able to predict
the structure of the disk at a higher resolution than can actually be seen. This is what the disk
would look like if observed with a telescope ten times bigger than the JCMT.

The irregular shape of the disk is the clue that it is likely to contain
planets. Although it is not possible to directly observe the planets,
they have created clumps in the dust disk around the star.

Similar history to our Solar System

The modelling suggests that the Neptune-like planet actually formed much
closer to the star than it is now. As it moved out to its current
wide orbit over about 56 million years, many comets were swept out with
it, causing the dust disk to be clumpy.

Exactly the same process is thought to have happened in our Solar System.
Neptune was forced away from the Sun because of the presence of
Jupiter orbiting inside it. So it appears that as well as having a Neptune-like
planet, Vega may also have a more massive planet in a smaller orbit
rather like Jupiter.

Submillimetre Astronomy

The James Clerk Maxwell
Telescope in Hawaii was used to take the image of the Vega dust disk. It is the worldâs largest
single-dish submillimetre telescope.
Credit:
Nik Szymanek

The submillimetre waveband lies between
the better known infrared and radio bands of the electromagnetic
spectrum.
It is the
waveband that best shows very cold gas and dust. One of the
most important discoveries of the last two decades was that many nearby
stars are
surrounded by cold dust disks that come from a reservoir of
comets orbiting further
from their stars than Neptune is from the Sun.

Finding planets around other stars is difficult

The light from a star is so bright that it swamps the light
coming from any planets. This means that astronomers have
to find clever
ways to
look for them.

The gravity of a large planet causes its star to wobble.
Astronomers can detect
this wobble.

Stars like the sun give out a lot of visible light and heat,
but at other wavelengths they hardly give out anything.
If you use
a telescope
sensitive
to these wavelengths
you can see the disk of comets, asteroids and dust which
orbit the star. Structure such as clumps in these disks is providing
the first
evidence
of relatively
small Neptune-like planets orbiting in these systems.